Nanosynthesis and Characterization of Cu1.8Se0.6S0.4 as a Potential Cathode for Magnesium Battery Applications.

Copper selenide (Cu-Se) and copper sulfide (Cu-S) are promising cathodes for magnesium-ion batteries. However, the low electronic conductivity of Cu-Se system results in a poor rate capability and unsatisfactory cycling performance. Mg-ion batteries based on the Cu-S cathode exhibited large kinetic barriers during the recharging process owing to the presence of polysulfide species. This work attempts to circumvent this dilemma by doping Cu1.8Se by sulfur, which replaces the selenium in the CuSe lattice to form Cu1.8Se0.6S0.4 nanocrystalline powder. The presence of sulfur will increase the electronic conductivity, and the presence of selenium will mitigate the effect of polysulfide species that hinder the kinetics of Mg2+. Herein, a Cu1.8Se0.6S0.4 nanocrystalline powder was synthesized by the solid-state reaction, yielding a highly pure and stoichiometric powder. The crystallographic structure of the nanopowder and the conversion-type storage mechanism have been attested via ex situ X-ray diffraction and energy-dispersive X-ray analysis. The nanocrystalline feature of Cu1.8Se0.6S0.4 was demonstrated by high-resolution transmission electron microscopy. An apparent surface morphology change during the charging/discharging process has been visualized by a field emission scanning electron microscope. Diffuse reflectance spectroscopy has discussed the variation of the band gap during charging and discharging. The full Mg/Cu1.8Se0.6S0.4 cells presented an initial discharge capacity of 387.99 mAh g-1 at a current density of 0.02 mA cm-2; moreover, they show moderate diffusion kinetics with DMg2+ ≈ 10-15 cm-2 s-1.

Unusual Activity of Rationally Designed Cobalt Phosphide/Oxide Heterostructure Composite for Hydrogen Production in Alkaline Medium.

Design and development of an efficient, nonprecious catalyst with structural features and functionality necessary for driving the hydrogen evolution reaction (HER) in an alkaline medium remain a formidable challenge. At the root of the functional limitation is the inability to tune the active catalytic sites while overcoming the poor reaction kinetics observed under basic conditions. Herein, we report a facile approach to enable the selective design of an electrochemically efficient cobalt phosphide oxide composite catalyst on carbon cloth (CoP-CoxOy/CC), with good activity and durability toward HER in alkaline medium (η10 = -43 mV). Theoretical studies revealed that the redistribution of electrons at laterally dispersed Co phosphide/oxide interfaces gives rise to a synergistic effect in the heterostructured composite, by which various Co oxide phases initiate the dissociation of the alkaline water molecule. Meanwhile, the highly active CoP further facilitates the adsorption-desorption process of water electrolysis, leading to extremely high HER activity.

Optically and Electrocatalytically Decoupled Si Photocathodes with a Porous Carbon Nitride Catalyst for Nitrogen Reduction with Over 61.8% Faradaic Efficiency.

The photoelectrochemical (PEC) approach is attractive as a promising route for the nitrogen reduction reaction (NRR) toward ammonia (NH3 ) synthesis. However, the challenges in synergistic management of optical, electrical, and catalytic properties have limited the efficiency of PEC NRR devices. Herein, to enhance light-harvesting, carrier separation/transport, and the catalytic reactions, a concept of decoupling light-harvesting and electrocatalysis by employing a cascade n+ np+ -Si photocathode is implemented. Such a decoupling design not only abolishes the parasitic light blocking but also concurrently improves the optical and electrical properties of the n+ np+ -Si photocathode without compromising the efficiency. Experimental and density functional theory studies reveal that the porous architecture and N-vacancies promote N2 adsorption of the Au/porous carbon nitride (PCN) catalyst. Impressively, an n+ np+ -Si photocathode integrating the Au/PCN catalyst exhibits an outstanding PEC NRR performance with maximum Faradaic efficiency (FE) of 61.8% and NH3 production yield of 13.8 µg h-1 cm-2 at -0.10 V versus reversible hydrogen electrode (RHE), which is the highest FE at low applied potential ever reported for the PEC NRR.

Electrocatalytic Water Oxidation by a Phosphorus-Nitrogen O═PN3-Pincer Cobalt Complex.

Water oxidation is a primary step in natural as well as artificial photosynthesis to convert renewable solar energy into chemical energy/fuels. Electrocatalytic water oxidation to evolve O2, utilizing suitable low-cost catalysts and renewable electricity, is of fundamental importance considering contemporary energy and environmental issues, yet it is kinetically challenging owing to the complex multiproton/electron transfer processes. Herein, we report the first cobalt-based pincer catalyst for catalytic water oxidation at neutral pH with high efficiency under electrochemical conditions. Most importantly, ligand (pseudo)aromaticity is identified to play an important role during electrocatalysis. A significant potential jump (∼300 mV) was achieved toward a lower positive value when the aromatized cobalt complex was transformed into a (pseudo)dearomatized cobalt species. The dearomatized species catalyzes the water oxidation reaction to evolve oxygen at a much lower overpotential (∼340 mV) on the basis of the onset potential (at a current density of 0.5 mA/cm2) of catalysis at pH 10.5, outperforming other Co-based molecular catalysts reported to date. These observations may provide a new strategy for the judicious design of earth-abundant transition-metal-based water oxidation catalysts.

Design and Mechanistic Study of Highly Durable Carbon-Coated Cobalt Diphosphide Core-Shell Nanostructure Electrocatalysts for the Efficient and Stable Oxygen Evolution Reaction.

The facile synthesis of hierarchically functional, catalytically active, and electrochemically stable nanostructures holds a tremendous promise for catalyzing the efficient and durable oxygen evolution reaction (OER) and yet remains a formidable challenge. Herein, we report the scalable production of core-shell nanostructures composed of carbon-coated cobalt diphosphide nanosheets, C@CoP2, via three simple steps: (i) electrochemical deposition of Co species, (ii) gas-phase phosphidation, and (iii) carbonization of CoP2 for catalytic durability enhancement. Electrochemical characterizations showed that C@CoP2 delivers an overpotential of 234 mV, retains its initial activity for over 80 h of continuous operation, and exhibits a fast OER rate of 63.8 mV dec-1 in base.